Hello,
NordVPN does not work with MTU lower then 1500 according to their support. I would assume, that you are trying to establish VPN connectivity using mobile interface.
If that is the case, I would like you to access router's WebUI, navigate to Network -> Interfaces, edit mobile settings, switch to Advanced settings tab and enter 1500 value to Override MTU field. Save the settings.
Otherwise, I would like you to send me a troubleshoot file in a private message. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
Best regards,